The gas phase fluidized bed reactors for which this invention is most useful are those such as described in US Patents to Noshay et al U.S. Pat. No. 4,482,687, Goode et al U.S. Pat. No. 4,803,251, Chirillo et al U.S. Pat. No. 4,855,370, Painter et al U.S. Pat. No. 5,428,118, and Chinh et al U.S. Pat. No. 5,733,510. These and other reactors to which our invention is applicable are characterized by a straight cylindrical section topped by a bulbous expanded section and are widely used for polymerizing .alpha.-olefins such as ethylene, propylene and others having up to about eight carbon atoms. They are large reactors having relatively thick steel walls, i.e. about two inches to about four inches thick.
The reactions conducted in such reactors generally call for polymerization of the monomer or monomers in the gas phase. A catalyst is also introduced, and, as polymerization proceeds, small particles of polymer product are formed, which are suspended in the gas as a fluid bed.
The polymerization process is exothermic, and accordingly it is common continuously to recycle a large portion of the gas to extract energy, i.e. the heat of reaction, from the system so the process can be controlled. As the amount of heat generated is directly related to the amount of polymer produced, the efficiency of heat removal is a prime determinant of productivity and yield.
Moreover, if the temperature in the reactor becomes too high, the particles will soften or even melt, and tend to stick together. In addition there is commonly a risk of a triboelectric effects among the particles and especially between the particles and the wall. Static charges are difficult to control and may be generated for reasons not completely understood, but their presence appears to be related to the temperature of the wall, the temperature of the gas in the fluidized bed, and the dew point of the gas. When the temperature of the interior wall becomes too high relative to the dew point of the gas, static charges are more likely to be present, leading to excess coating formation on the wall and the undesirable phenomenon of sheeting.
Since large gas phase reactors are commonly outdoors and exposed to the elements, they have often been rained upon, but no one, to our knowledge, has attempted to adapt the operation of the reactor to accommodate the effect of rain on the internal temperature of the reactor. Attempts to control the wall temperature include the cooling tube disclosed in Japanese patent application 9-302008, and the cooling jacket described in Japanese patent applications 9-249703 and 7062009 A. All three of these disclosures speak of cooling the inner wall to a temperature below the dew point of the gas phase by cooling the outside of the reactor, but such methods require expensive cooling equipment and circulating apparatus. Negative factors in the use of various types of cooling jackets include the difficulty of sealing them, and the common presence of manways and instrumentation which could be covered by the jacket. A major safety factor is that a reactor leak may go undetected, underneath the jacket, so that explosive gases are concentrated in the cooling jacket or delivered with the used water to a drain basin or the like.
Wall cooling has been used in fluidized bed combustion systems--see U.S. Pat. Nos. 5,025,755 and 4,944,150.
U.S. Pat. No. 3,254,070 illustrates an early polymerization reactor equipped with a cooling jacket 56 for removing heat of reaction.
Canadian patent 709,470 shows a water jacket for cooling a gas phase polymerization process reactor, as a supplement to recycling unreacted monomer through a heat exchanger to reduce the temperature of the gas.
The reader may also be interested in Hussein U.S. Pat. No. 4,981,929 in which the dew point of the gas phase is controlled through the addition of a nonreactant gas recirculated with the unreacted monomer. The reaction temperature and the dew point are caused to approach one another either by such an addition to elevate the dew point or by lowering the temperature of the reaction. In the preferred mode, the polymerization is conducted at a temperature 0.1 to 5.0 degrees Centigrade above the dew point of the cycle gas in the reactor.